Selective deposition of amorphous silicon film seeded in a chlorine gas and a hydride gas ambient when forming a stacked capacitor with HSG

ABSTRACT

A polysilicon film is formed with enhanced selectivity by flowing chlorine during the formation of the film. The chlorine acts as an etchant to insulative areas adjacent polysilicon structures on which the film is desired to be formed. Bottom electrodes for capacitors are formed using this process, followed by an anneal to create hemishperical grain (HSG) polysilicon. Multilayer capacitor containers are formed in a non-oxidizing ambient so that no oxide is formed between the layers. The structure formed is planarized to form separate containers made from doped and undoped amorphous silicon layers. Selected ones of undoped layers are seeded in a chlorine containing environment and annealed to form HSG. A dielectric layer and second electrode are formed to complete the cell capacitor.

FIELD OF THE INVENTION

The present invention relates to semiconductor field effect transistors, and in particular to selective depostion on polysilicon surfaces.

BACKGROUND OF THE INVENTION

Dynamic random access memories (DRAMs) are a widely used form of semiconductor memory. DRAMs are composed of a memory cell array and peripheral circuitry. Each memory cell array is formed of a plurality of memory cells for storing information. Typical memory cells are formed with a transistor for accessing a capacitor that stores charge. Of primary concern is maximizing the storage capacitance of each memory cell capacitor. This need is particularly acute in light of the demand for high density DRAMs, without increasing the chip area required to form the cell and, preferably, allowing a decrease in the chip area per cell.

One way to achieve greater capacitance per cell is to increase the surface area of the capacitor electrodes without increasing the respective cell area. As can be seen from the following equation, capacitance, C, is roughly determined by the thickness of the capacitor insulator (t_(ox)), the surface area of the capacitor electrodes (A), and the dielectric constant of the capacitor insulator (ε).

    C=(ε·A)/t.sub.ox

Increasing thc surface area of the capacitor electrodes by forming the storage capacitor in a container like shape is well known in the art. To further increase circuit density in DRAMs stacked capacitors are used. These capacitors are actually stacked on top of the substrate, which may or may not include access devices. Two or more layers of a conductive material called electrodes formed of polysilicon or poly are deposited over the substrate with dielectric layers sandwiched between each electrode.

U.S. Pat. No. 5,340,765 to Dennison et al., herein incorporated by reference, describes a method for further increasing the surface area of a bottom electrode of such capacitors by forming the electrode surfaces with hemispherical grained polysilicon (HSG). First, a portion of an oxide layer covering access circuitry on a semiconductor wafer is removed to form a container. A bottom electrode is then formed by growing an amorphous silicon layer. The amorphous silicon layer is then seeded by flowing silane or disilane at elevated temperatures.

After seeding, the wafer is annealed to form HSG on both sides of the doped silicon layer. Formation of a dielectric layer and top capacitor plate complete the capacitor formation.

While the above method creates a roughened surface on the capacitor electrode plate to increase capacitance per unit area, it can lead to undesired deposits forming on oxide, BPSG or other insulator between structures where silicon is desired to be deposited. Such conductive deposits are referred to as stringers which are caused by residual poly film deposited on oxide surfaces that short nearby storage cells or other structures where it is desired to form the poly film. Further steps, such as etches are then required to eliminate such shorting.

There is a need to form poly films in a selective manner. There is a need to form selective poly films in a manner which reduces the need for further processing steps to remove undesired residual poly films. There is yet a further need to form selective poly films without forming shorts between structures on which the film is desired. There is a further need to form such poly films only where formation of HSG is desired. With the rapid decrease in device size, coupled with the demand for increased performance, a new, efficient technique must be found to provide an increase in the capacitance per unit area in DRAM memory cells without causing shorts between cells.

SUMMARY OF THE INVENTION

A chlorine containing gas mixture is flowed during seeding of polysilicon layers to enhance formation of films in a selective manner. The chlorine containing gas enhances selectivity of film formation such that film forms on the polysilicon layers while formation of the film on adjacent insulative layers is inhibited.

In one embodiment of the invention, polysilicon layers of a capacitive storage cell are seeded with hydride gases such as silane, disilane or trisilane while a chlorine containing gas, such as HCl is introduced. The seeding is performed at a temperature of approximately 500 to 800 degrees Celsius. Pressures of up to near atmosphere may be used while still maintaining high selectivity. The seeding is followed by an anneal at a temperature of approximately 500 to 800 degrees Celsius. No retooling from prior HSG formation processes is required, and the higher selectivity provides the ability to modify other process parameters to acheive other goals while still maintaining adequate selectivity.

In a further embodiment, polysilicon structures supported by a substrate are separated by adjacent insulative layers such as oxides or BPSG. The structures are selectively seeded by silane or disilane in the presence of a chlorine containing gas to selectively form a polysilicon film. The use of the chlorine containing gas greatly inhibits the formation of the film between the structures, greatly reducing the chance of a short between the structures.

In yet a further embodiment, a DRAM device comprises multiple stacked capacitive cells having bottom electrodes with selectively seeded HSG. The use of chlorine during seeding of the bottom electrodes provides an increase in the capacitance per unit area in the DRAM memory cells without causing shorts between cells.

In a further variation, polysilicon structures supported by a substrate are separated by adjacent insulative layers such as oxides or BPSG, and the structures are selectively seeded in the presence of an etchant which selectively etches the insulative layers. This inhibits the formation of polysilicon nucleation sites on the insulative layers, resulting in greatly reduced formation of shorts between the structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are cross sectional views of a polysilicon structure supported by a substrate on which a selective film is formed.

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, and 2H are cross-sectional representations illustrating steps of forming selective HSG on a polysilicon structure in accordance with the present invention.

DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. Numbering in the Figures is usually done with the hundreds and thousands digits corresponding to the figure number, with the exception that the same components may appear in multiple figures. Scaling in the Figures does not represent precise dimensions of the structures illustrated.

Wafer and substrate are used interchangeably to refer to supporting semiconductor structures during processing. Both are to be understood as including silicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI) technology, doped and undoped semiconductors, epitaxial layers of a silicon supported by a base semiconductor, as well as other semiconductor structures well known to one skilled in the art. Furthermore, when reference is made to a wafer or substrate in the following description, previous process steps may have been utilized to form regions/junctions in the base semiconductor structure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

In FIG. 1A, an amorphous silicon "U" shaped structure referred to as container 110 is formed on a substrate 100 in a well known manner. It is formed in an etched area of an insulative layer 106, which has been etched back to expose exterior sidewalls of container 110. A second container, partially shown at 112 is separated from the first container 110 by insulator 106. Insulator 106 is an oxide or BPSG, or other insulator which inhibits electrical conduction between containers 110 and 112.

It is desired to form a selective silicon based film 116, as shown in FIG. 1B, on the containers 110 and 112 without forming any conductive path on the insulator 106 between the containers. To do this, a chlorine containing gas mixture is added to dilute hydride gases normally used during seeding of the containers. This results in a thin polysilicon film being formed on exposed container surfaces, while such a film is inhibited from forming on insulator 106.

In one embodiment, the hydride gas comprises silane, disilane or trisilane flowed at least 1 sccm, but preferably within a range of 5 to 20 sccm, with a nitrogen ambient flowed at a selected rate depending on desired dilution such as 500 to 1000 sccm. Other inert ambient gases such as argon may also be used. Chlorine is then provided following stabilization of the hydride flow and the temperature of the system is raised in steps to approximately 500-800 degrees Celsius, which is low enough to prevent active formation of HSG. Since chlorine acts as an etchant on the insulative layer 106, it helps prevent the formation of a silicon film on the insulative layer 106.

In a further embodiment, a carrier gas such as H₂ or N₂ is used, and HCl is flowed following stabilization of the hydride flow during seeding of the container structures.

Pressures in which the seeding is conducted range from 1.0×10⁻⁴ to approximately atmospheric pressure, using a 10% carrier gas of H₂ or N₂ with disilane flows ranging from 5 sccm to 20 sccm at a temperature of at least approximately 550 to 660 degrees Celsius or within a range of about 500-800 degrees Celsius for a variable time as desired. In one embodiment, the time is about 70 seconds.

Following the seeding step, an anneal is performed to create HSG on the container structures. The anneal is commonly performed at a temperature of at least approximately 500 degrees Celsius or within a range of 600 to 700 degrees Celsius in the same pressure ranges for about 40 seconds to form HSG. The time period may range from 1 to 60 seconds or more. Experimental results following the above process variations have shown that use of H₂ as a diluting/carrier gas results in better selectivity as reflected in increased resistance exhibited between adjacent containers over prior methods.

In a further embodiment as represented in FIGS. 2A-H, formation of a stacked capacitor bottom electrode is described. The steps used to form the electrode include the use of chlorine in the seeding step prior to formation of HSG to increase the capacitance. In FIG. 2A a multi layer insulative sidewall comprising a support layer 202 of undoped oxide, BSG, PSG or TEOS which is formed on a substrate 200 in a known manner. The substrate 200 also contains other circuitry commonly used to form a dynamic random access memory (DRAM). The support layer 202 is followed by formation of an etch stop layer 220 comprising nitride. The etch stop layer 220 is further followed by formation of a BPSG layer 222, completing the multi layer sidewall. An opening 203 is then etched in the sidewall. The opening may extend all the way to the substrate, and may expose active areas formed thereon, or may also be formed within the sidewall layer above the substrate, perhaps contacting circuitry formed in the sidewall layer. Alternatively, a plug is formed in the bottom of the container to contact desired active areas.

Following formation of the opening 203, a first layer of undoped or substantially undoped amorphous polysilicon 204 having a thickness approximately between 40 and 500 angstroms is formed over the insulative layer, as shown in FIG. 2B. Typically, the undoped amorphous polysilicon 204 is formed at temperatures between approximately 450 and 580 degrees Celsius. Because it is formed at a low pressure of 1 atmosphere or below, such as within a range of approximately 10⁻³ to 10⁻¹ Torr in hydrogen or other inert ambient having a base pressure of approximately 10⁻⁸ Torr, but within a range of approximately 10⁻⁸ to 10⁻¹ Torr, the undoped amorphous polysilicon 204 is formed with a relatively low surface roughness. Subsequently, a heavily doped n+ amorphous silicon layer 206 having a thickness between 100 and 1000 angstroms is deposited on the first undoped layer 204, as shown in FIG. 2C. The layer 206 may be n or p doped. Common dopants for layer 206 comprise n or p-type dopants such as arsenic, phosphorous or boron. The doped layer 206 is formed at a temperature between approximately 450 and 600 degrees Celsius with a doping level between 5×10¹⁸ and 5×10²² cm⁻³ at a pressure range of approximately 10⁻⁸ to 10⁻¹ Torr.

The heavily doped layer 206 is followed by a second layer of undoped or substantially undoped amorphous silicon 208, which is formed on the doped amorphous silicon layer 206 under similar conditions and using the same or similar parameters as used when forming first undoped layer 204, as shown in FIG. 2D. The substantially undoped layers 204, 208 provide good nucleation sites for subsequent seeding used to form hemispherical grained polysilicon (HSG). The layers are substantially amorphous to aid in the formation of HSG, but need not be completely amorphous. The silicon layers 204, 206, and 208 are typically formed with silane or disilane. However, organo- and other hydride precursors can be used instead. Processing is performed in a hydrogen atmosphere to prevent an undesirable insulating layer, of oxide for example, from forming on or between the silicon layers during formation. The silicon layers are formed in a single wafer tool so that ambients can be changed quickly, which helps prevent oxidation between layers. Batch and batch cluster tools may also be used.

Next, the wafer is planarized, such as by chemical mechanical planarization for example, to remove undesired lateral portions of the deposited silicon layers 204, 206, 208, as shown in FIG. 2E. Subsequently, BPSG layer 222 is removed as by etching to expose the exterior sidewalls of the bottom electrode in the shape of a container 210 in FIG. 2F. Layer 222 should have good etch selectivity with respect to etch stop 220, which is formed of nitride or other desired dielectric material that functions as an etch stop. Optionally, layer 222 has sufficient etch selectivity with respect to support layer 202 such that no separate etch stop layer is required. By modifying the thicknesses of these sidewall layers 202, 220 and 222, the amount of support provided to the resulting container can be varied as desired. Such a multi layer insulator may also be used to provide support to other capacitor containers formed in different manners.

The sides of the resulting container comprise pillars of doped silicon 206 surrounded by layers of "paint" comprising undoped silicon layers 204 and 208. Because the first amorphous silicon layer 204 has a relatively low surface roughness, most of the insulator layer 222 is removed. Thus, capacitors can be separated by a pitch of less than two tenths of the diameter of a container, such as 0.18 microns in current high density DRAMs.

At this point, a native oxide remains on the exterior exposed pillars as indicated by the broken line in FIG. 2F. The native oxide is selectively removed by lithography and wet or dry etching, such as with a combination of hydrofluoric acid, chlorine, and ultraviolet light as represented in FIG. 2G. Removal of the native oxide promotes enhanced HSG formation in the next step.

To selectively create HSG, the thin, smooth, amorphous silicon layers 204 and 208 are then seeded with a chemical species where they are exposed, such as germanium, silicon, or titanium, in combination with a hydride gas such as silane, disilane or trisilane at different partial pressures. In one embodiment, germanium seeding is provided at approximately greater than 1%, in an approximate range of 1 to 30%. The seeding is performed in a rapid thermal or low pressure chemical vapor deposition tool. The silane or disilane is added at a rate as previously described. The species are added at a pressure of approximately one atmosphere or less.

Chlorine is then provided following stabilization of the hydride flow and the temperature of the system is raised in steps to approximately 500-800 degrees Celsius, which is low enough to prevent active formation of HSG. Since chlorine acts as an etchant on the insulative layer 202, it helps prevent the formation of a silicon film on the insulative layer 202 by removing any residual silicon that forms. It further may have a roughening effect by inhibiting formation of extra silicon between grains of the HSG that forms in further steps.

In a further embodiment, a carrier gas such as H₂ or N₂ is used, and HCl is flowed following stabilization of the hydride flow to seed the container structures.

Pressures in which the seeding is conducted range from 1.0×10⁻⁴ to 1 atmosphere, using a 10% carrier gas of H₂ or N₂ with disilane flows ranging from 5 sccm to 20 sccm at a temperature of about 500-800 degrees Celsius for a variable period of time, such as about 70 seconds.

Subsequently, the DRAM 200 is annealed to convert the undoped layers 204, 208 into HSG as indicated in FIG. 2H. The anneal is performed at a temperature in the range of approximately 450 to 700 degrees C, sufficient to cause at least partial migration of dopants to undoped layers. The resulting structure comprises pillars of doped silicon surrounded by layers of HSG having dopants which have migrated from the doped pillars. In one embodiment, layer 206 is n doped, and may also be seeded with germanium.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. For example, the shape of the storage electrode may be a plain cubic structure, a cylindrical structure, a fin-type structure, or a stacked trench type structure. Furthermore, the present invention describes the formation of a three-dimensional container for a bottom electrode of a stacked capacitor. However, other electrode structures, including a ministack, and honeycomb, known to those skilled in the art, can be formed in a similar manner. Still further, the method of selective deposition may be used in memory devices other than DRAMs and for selective deposition on other silicon structures structures, such as conductors while preventing formation of polysilicon on oxide surfaces. It may be used to produce capacitors used in general circuitry, and not for storage of data. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 

What is claimed is:
 1. A method for forming a capacitor in an insulative layer supported by a substrate, the method comprising the steps of:removing a first portion of the insulator layer to form an opening; forming a first layer of undoped amorphous silicon on the insulative layer; forming a layer of doped amorphous silicon on the first layer of silicon; forming a second layer of undoped amorphous silicon on the doped layer of silicon; forming containers in the insulative layer (by planarization) comprising pillars of doped silicon painted with the layers of undoped amorphous silicon; removing selected portions of the insulative layer to expose at least a portion of exterior sidewalls of the containers; and seeding the first and second amorphous silicon layers in a seeding environment consisting of a carrier with a dilute hydride gas and a chemical species at a low temperature and pressure to form a seeded silicon coating; preventing the silicon coating from building up on the insulative layer by adding chlorine to the gases used in the preceding seeding step after stabilization of the hydride flow; and annealing the seeded amorphous silicon layers at a higher temperature to convert the first and second layers to HSG while continuing the flow of the carrier gas.
 2. The method of claim 1 where the seeding environment further contains a hydride of silicon.
 3. The method of claim 2 where the low temperature is less than about 800° C.
 4. The method of claim 2 where the higher temperature exceeds about 500° C.
 5. The method of claim 2 where the low pressure is in the range of about 10⁻⁴ to 1 Torr.
 6. The method of claim 1 further including the step of removing an oxide formed on the exterior sidewalls before the seeding step.
 7. The method of claim 6 wherein the step of removing the oxide formed on the exterior sidewalls is selectively performed by lithography and wet or dry etching.
 8. The method of claim 1 where the adding of chlorine in the step of preventing the buildup of silicon on the insulative material is performed by flowing HCl in combination with a stabilized carrier gas and hydride gas.
 9. The method of claim 8 wherein the carrier gas is H₂ or N₂.
 10. A method for forming capacitors in an insulative layer supported by a substrate, the method comprising the steps of:forming multiple containers in the insulative layer comprising pillars of doped silicon painted with layers of undoped amorphous silicon; removing selected portions of the insulative layer to expose exterior sidewalls of the containers; seeding the amorphous silicon layer s of the containers in low-pressure, low temperature environment containing chlorine to prevent silicon formation between containers on the insulative layer; and annealing the containers at a higher temperature to convert the seeded amorphous layers to HSG.
 11. The method of claim 10 where the environment during seeding further contains a gas selected from the group consisting of silane, disilane, and trisilane.
 12. The method of claim 10 where the environment during seeding further contains a carrier gas.
 13. A method of forming an HSG polysilicon film on silicon structures supported by a substrate and separated by insulative material, comprising:forming a thin, amorphous silicon layer on the underlying silicon structures; seeding the amorphous silicon layer with a chemical species in combination with a hydride gas flow to form a doped silicon layer on the underlying silicon; adding a chlorine-containing gas during seeding to retard any bonding of silicon to the insulative material; heating the substrate during seeding to a temperature below the temperature sufficient to permit active formation of HSG on the silicon layer; and thereafter, vacuum annealing the layer at a higher temperature and for a time duration sufficient so as to convert the doped silicon layer to an HSG polysilicon film on the silicon structure.
 14. The method of claim 13 where the seeding is performed at a low ambient pressure.
 15. The method of claim 24 where the seeding is performed at a pressure below about 10⁻¹ Torr.
 16. The method of claim 13 where the chlorine-containing gas is added after stabilization of the hydride gas flow.
 17. The method of claim 13 where the chemical species is selected from the group consisting of germanium, silicon, and titanium.
 18. The method of claim 13 where the hydride gas is selected from the group consisting of silane, disilane, and trisilane.
 19. The method of claim 18 where the hydride gas is flowed at the rate of at least 1 sccm.
 20. The method of claim 19 where the hydride gas is flowed at the rate in the approximate range of 5-20 sccm.
 21. The method of claim 13 where the seeding is performed by rapid thermal chemical vapor deposition.
 22. The method of claim 13 where the seeding is performed by rapid low-pressure vapor deposition.
 23. The method of claim 22 where annealing is performed at a temperature in the approximate range of 600-700° C.
 24. The method of claim 13 where the chlorine-containing gas is selected from the group comprising Cl₂ and HCl.
 25. The method of claim 13 where the heating of the substrate consists of raising the temperature of the substrate to approximately 500-800° C.
 26. The method of claim 25 where the temperature to which the substrate is heated is raised in steps.
 27. The method of claim 13 where annealing is performed at a temperature of at least 500° C.
 28. A method of forming capacitor electrodes separated by insulative material on a substrate, comprising:forming an electrode container having pillars of heavily doped silicon sandwiched by layers of substantially undoped amorphous silicon; seeding a chemical species onto the layers of amorphous silicon at a low temperature and low pressure wherein the seeding is performed in the presence of a hydride gas and a chlorine gas to prevent build-up of silicon on the insulative material; annealing the layers at a higher temperature so as to convert the layers to HSG polysilicon films.
 29. The method of claim 28 where the low pressure is less than atmospheric pressure.
 30. The method of claim 29 where the low temperature is less than about 800° C.
 31. The method of claim 28 where the hydride gas is selected from the group consisting of silane, disilane, and trisilane.
 32. The method of claim 28 where the chlorine gas is selected from the group consisting Of Cl₂ and HCl.
 33. A method of forming HSG layers separated by insulative material on a substrate, comprising:forming HSG precursor layers of amorphous silicon; seeding a chemical species onto the layers of amorphous silicon at a low temperature and low pressure wherein the seeding is performed in the presence of a hydride gas and a chlorine gas which prevents build-up of silicon on the insulative material; converting the HSG precursor layers to HSG layers.
 34. The method of claim 33 wherein the converting of the HSG precursor layers is performed by annealing the HSG precursor layers at an elevated temperature so as to convert the HSG precursor layers to HSG layers.
 35. The method of claim 34 where the low pressure is less than atmospheric pressure.
 36. The method of claim 35 where the low temperature is less than about 800° C.
 37. The method of claim 34 where the hydride gas is selected from the group consisting of silane, disilane, and trisilane.
 38. The method of claim 34 where the chlorine gas is selected from the group consisting of Cl₂ HCl.
 39. A method of forming HSG layers separated by insulative material on a substrate, comprising:forming HSG precursor layers of amorphous silicon; seeding a chemical species onto the layers of amorphous silicon at a low temperature and low pressure wherein the seeding is performed in the presence of a hydride gas diluted with a chlorine gas which prevents build-up os silicon on the insulative material; and forming the HSG layers by converting the HSG precursor layers to HSG layers.
 40. A method for forming a capacitor in an insulative layer supported by a substrate, the method comprising the steps of:removing a first portion of the insulator layer to form an opening; forming a first layer of undoped amorphous silicon on the insulative layer and the opening; forming a layer of doped amorphous silicon on the first layer of silicon; forming a second layer of undoped amorphous silicon on the doped layer of silicon; forming a container in the insulative layer by planarization to remove lateral portions of the first and second layers of undoped amorphous silicon and the layer of doped amorphous silicon; removing selected portions of the insulative layer to expose at least a portion of exterior sidewalls of the container which projects from the surface of the insulative layer as a pillar of doped silicon surrounded with layers comprising undoped silicon; and seeding the first and second amorphous silicon layers in a seeding environment consisting of a carrier gas with a dilute hydride gas and a chemical species at a first temperature and pressure to form a seeded silicon coating on the silicon layers; preventing the silicon coating from building up on the insulative layer by adding chlorine to the gases used in the preceding seeding step after stabilization of the hydride flow; and annealing the seeded amorphous silicon layers at a second temperature which is higher than the first temperature thereby to convert the first and second layers to HSG while continuing the flow of the carrier gas.
 41. A method for forming capacitors in an insulative layer supported by a substrate, the method comprising the steps of:forming multiple containers in the insulative layer, each container comprising a pillar of doped silicon sandwiched between with layers of substantially undoped amorphous silicon; removing selected portions of the insulative layer to expose exterior sidewalls of the containers; seeding the amorphous silicon layers of the containers in a low-pressure, low-temperature environment containing chlorine to prevent silicon formation between containers on the insulative layer; and annealing the containers at a higher temperature to convert the seeded amorphous layers to HSG.
 42. The method of claim 41 where the heavily doped silicon is doped with a doping level between 5×10¹⁸ and 5×10²² cm⁻³.
 43. The method of claim 41 where the low temperature is between about 450 and 600 degrees Celsius, the higher temperature is 500 to 800 degrees Celsius and the low pressure is about 1×10⁻⁴ to 1 atmosphere.
 44. A method of forming capacitor electrodes laterally separated by insulative material on a substrate, comprising:forming pillars of heavily doped silicon sandwiched by layers of substantially undoped amorphous silicon; seeding a chemical species onto the layers of amorphous silicon at a first temperature and low pressure wherein the seeding is performed in the presence of a hydride gas and a chlorine gas to prevent build-up of silicon on the insulative material; annealing the layers at a second temperature, higher than the first temperature, thereby converting the layers to HSG polysilicon films.
 45. The method of claim 44 where the low pressure is less than atmospheric pressure.
 46. The method of claim 45 where the first temperature is less than the temperature at which active formation of HSG begins.
 47. The method of claim 45 where second temperature is above about 450 to 700 degrees Celsius.
 48. The method of claim 45 where heavily doped silicon has a doping level of between 5×10¹⁸ and 5×10²² cm⁻³.
 49. The method of claim 48 wher the hydride gas is selected from the group consisting of silane, disilane, and trisilane.
 50. The method of claim 48 where the chlorine gas is selected from the group consisting of Cl₂ and HCl. 